Radiation therapy has been employed to treat tumorous tissue. In radiation therapy, a high energy beam is applied from an external source towards the patient. The external source produces a collimated beam of radiation that is directed into the patient to the target site. The dose and placement of the dose must be accurately controlled to ensure that the tumor receives sufficient radiation, and that damage to the surrounding healthy tissue is minimized.
Existing radiotherapy systems use electrons to generate the radiation beam. In such systems, the ability to control the dose placement is limited by the physics of the beam, which necessarily irradiates healthy tissue on the near-side and far-side of a target region as it passes through the patient. Thus, it may be desirable to use protons as the source of the radiation. By controlling the energy of the protons, the protons will stop at a precise location within the patient. In this way, tissue on the far-side of the target region does not receive any radiation dose. Further, because the dose provided by a proton is concentrated at a “Bragg peak” around the area where the proton stops, the dose to healthy tissue on the near-side of the target region may also be reduced.
In proton therapy, monitor unit(s) (MU) is a measure of dose, or an amount of radiation units produced by a machine. MU may be determined during treatment planning for planning purpose, and/or during actual treatment for verification of delivery of dose. Previously, in order to calculate monitor units, it was necessary to measure the output for each field before applying it for a patient treatment. A first approach to the calculation of monitor units was published in “Monitor unit calculations for range-modulated spread-out Bragg peak fields” Phys. Med. Biol. 48 2797-2808 by Kooy et al., 2003, and in “The prediction of output factors for spread-out proton Bragg peak fields in clinical practice” Phys. Med. Biol. 50 5847-5856 by Kooy et al., 2005.
Kooy's method has several disadvantages. First, the method is based on a theoretical model of a modulated proton spread-out Bragg peak (SOBP). It is therefore limited to MU calculation for flat SOBPs. Furthermore, it uses some fiddling factors, which must be obtained from a large collection of measurements. These fiddling factors depend on the details of the beamline design of each machine and on different settings of the same beamline design. Also, another method that has been employed for calculating MU may lead to large errors for lower ranges of the beam. These errors are related to a change of the entrance dose with respect to the height of the Bragg peak when changing the amount of absorber material in the beamline. In particular, the gradient in depth dose distribution is increasing with depth (or decreasing with residual range of the beam between its entry point and the Bragg peak). However, it only starts to play a major role at residual ranges below about 15 cm. The effect of this gradient in the depth dose on the MU factor is not addressed before, thereby leading to errors in the calculated MU for lower ranges of the beam. Also, there is some range shift in-between the monitor chamber and isocenter due to air or other materials, which was not considered before. For example, a range shift between the monitor and isocenter of 2-3 mm may contribute to 10% error in the calculated MU for a residual range of 2 cm.
For the foregoing reasons, it would be desirable to have systems and methods for determining monitor units that does not require a lot of calibration measurements to find machine specific factors. It would also be desirable to have systems and methods for determining monitor units that are more accurate for lower (residual) ranges of energy.